Smoke detector with external sampling volume using two different wavelengths and ambient light detection for measurement correction

ABSTRACT

In accordance with certain embodiments, a smoke detector determines the presence of smoke particles outside its housing based on measurements of light detected at different wavelengths and corrected based on an ambient light level.

RELATED APPLICATIONS

This application is (a) a continuation-in-part of U.S. patentapplication Ser. No. 13/799,816, filed Mar. 13, 2013, which claims thebenefit of and priority to U.S. Provisional Patent Application No.61/639,935, filed Apr. 29, 2012, (b) a continuation-in-part of U.S.patent application Ser. No. 13/799,826, filed Mar. 13, 2013, whichclaims the benefit of and priority to U.S. Provisional PatentApplication No. 61/639,935, filed Apr. 29, 2012, and (c) acontinuation-in-part of U.S. patent application Ser. No. 13/800,071,filed Mar. 13, 2013, which claims the benefit of and priority to U.S.Provisional Patent Application No. 61/639,935, filed Apr. 29, 2012. Theentire disclosure of each of these applications is hereby incorporatedherein by reference.

FIELD OF THE INVENTION

In various embodiments, the present invention generally relates to smokedetectors and, in particular, to such detectors having external samplingvolumes.

BACKGROUND

A smoke detector with an external sampling volume operates by emittinglight outside its housing and detecting light scattered back into thehousing by smoke particles located within the sampling volume. Smokedetectors with an external sampling volume have several importantbenefits over conventional ionization and photoelectric smoke detectors.First, by eliminating the internal sensing chamber and the slowaccumulation of smoke particles therein, the lag time between when athreshold smoke density is reached outside the detector and when thesmoke detector responds is substantially eliminated. This increases theAvailable Safe Egress Time (ASET), the time available for occupants tosafely evacuate a building before the fire renders evacuationimpossible. Second, by obviating the need for the entry of smokeparticles into the housing, the entirety of the smoke detector may bemounted within an opening in a ceiling or wall, such that there isminimal protrusion outward from the surface; such flush mounting of thesmoke detector creates an aesthetically pleasing appearance. Third, thesmoke-detecting element may be fully tested. In conventional ionizationand photoelectric smoke detectors equipped with a test feature, thetesting mechanism tests the electrical circuitry only, but in smokedetectors with an external sampling volume, the operation of thesmoke-detecting element may be tested by inserting an object into thesampling volume.

Despite these benefits, smoke detectors with an external sampling volumehave not been widely deployed. One reason is the difficulty of thesesmoke detectors to isolate the signal generated by the scattered lightfrom the signal generated by ambient light, especially when there is achange in the ambient light level. Another reason is the difficultythese smoke detectors have distinguishing smoke particles from nuisanceparticles or other objects. The ambiguity in both cases may lead tofalse alarms when a nuisance source is present or a lack of responsewhen a fire source is present.

Accordingly, there is a need for smoke detectors with an externalsampling volume, and related detection techniques, which can reject theinfluence of ambient light and distinguish smoke particles from nuisanceparticles and objects.

SUMMARY

In accordance with various embodiments of the present invention, a smokedetector uses a proximity sensor (or multiple components collectivelyproviding the functionality of a proximity sensor) to detect thepresence of smoke outside the detector. The proximity sensor generallyoperates by emitting a beam of light and detecting any scattered orreflected signal from an object located within a specified range. Theproximity sensor features at least one light detector (which istypically but not necessarily embedded in the proximity sensor), alongwith control circuitry and signal processing circuitry. At least onelight emitter may also be embedded in the proximity sensor or may bediscrete but externally driven by the proximity sensor. The smokedetector also uses an ambient-light sensor to measure and compensate forthe ambient light level. The ambient-light sensor features at least onelight detector (which is typically but not necessarily embedded in theambient-light sensor), along with control circuitry and signalprocessing circuitry. The ambient-light sensor may be separate from theproximity sensor, or it may be part of (and even embedded within) theproximity sensor, in which case the ambient-light sensor and proximitysensor may use a common light detector. Alternative embodiments of theinvention utilize a discrete light emitter and light detector in placeof the proximity sensor without altering the functionality of the smokedetector. As utilized herein, a “light detector” is a discrete orembedded electronic component that registers the presence of and/ormeasures a property of light (e.g., luminance, wavelength, etc.) when itis illuminated by the light.

In accordance with various embodiments of the invention, the proximitysensor is disposed inside the housing of the smoke detector beneath anopening. The opening may or may not be covered by a window that is atleast partially transparent to the emitted light. Most of the emittedbeam passes through the opening to the environment outside the smokedetector. The region outside the smoke detector but within the specifiedrange of the proximity sensor (or other discrete components describedherein) is defined herein as the “external sampling volume.” If smoke oran obstruction enters the external sampling volume, the signal generatedby the proximity sensor will increase. In the case of smoke, theincrease in signal arises from scattering of the emitted beam by thesmoke particles. In the case of an obstruction, the increase in signalarises from the reflection of the emitted beam off of the obstruction.Because the proximity sensor is optically exposed to the outsideenvironment via the opening (or the window), its signal may also beincreased or decreased by ambient light incident upon the proximitysensor. Ambient light, as utilized herein, is any light that enters theexternal sampling volume or the housing that did not originate from alight emitter inside or associated with the smoke detector. Exampleambient light sources include sunlight or light from incandescent,fluorescent, halogen, or LED light bulbs.

An evaluation circuit may periodically or continuously analyze thesignal to determine whether an obstruction, smoke, or system fault ispresent. Since reflection by an obstruction typically produces adistinctly stronger signal than scattering by smoke particles, anobstruction threshold is typically set higher than the maximum possiblesignal generated by smoke scattering. If the signal exceeds theobstruction threshold for a pre-determined amount of time, anobstruction alarm may be activated. This pre-determined delay typicallyeliminates unwanted alarms from fleeting events such as an insectpassing through the external sampling volume.

The smoke threshold is generally set lower than the obstructionthreshold but higher than the background signal, and the smoke thresholdmay correspond to the signal generated for a given smoke density outsidethe detector. If additional sensors are incorporated in the smokedetector, such as gas or heat sensors, the smoke threshold may bedecreased with increasing signal from these sensors, as the signal fromthe additional sensor(s) may provide faster activation and greaterdiscrimination from nuisance sources (i.e., false alarms). An advantageof embodiments of the present invention is that the proximity sensordirectly measures the smoke density outside the smoke detector, whichsubstantially reduces the lag time compared to a conventional ionizationor photoelectric smoke detector.

The operation of the smoke detector may be manually tested by insertingan object, such as a hand or broom handle, into the external samplingvolume to activate the obstruction alarm after a pre-determined delayhas elapsed. Likewise, inserting an object into the external samplingvolume while an alarm is activated may temporarily silence the alarm.

Embodiments of the invention also distinguish between smoke particlesand nuisance particles based at least in part on specific interactionsbetween such particles and multiple different wavelengths of light. (Asutilized herein, “nuisance particles” broadly refers to vapors orairborne particulates not originating from a fire and that typicallyhave average diameters larger (e.g., at least ten times larger and/or atleast one micron in diameter) than typical smoke particles. Non-limitingexamples of nuisance particles are steam, cooking aerosols (e.g.,vegetable oil, toast, hamburger, bacon, etc.), powder, and dust (e.g.,cement dust).) Because nuisance particles typically are larger thansmoke particles, they will tend to scatter light of various wavelengthsdifferently. Thus, the scattering behavior over multiple wavelengths maybe utilized to distinguish nuisance particles from smoke particles.Furthermore, embodiments of the invention also correct light-detectionsignals received from the external sampling area based on (1) thespecific behavior and properties of the light emitter(s) being utilizedand (2) the amount of ambient light. In this manner, smoke detectors inaccordance with embodiments of the present invention more correctlyidentify airborne particles and obstructions without the false positivealarms of conventional systems.

In an aspect, embodiments of the invention feature a method of smokedetection utilizing a smoke detector comprising or consistingessentially of (a) a housing, (b) one or more light emitters, and (c)one or more light detectors. At a first time, a first measurement oflight including a first wavelength originating outside the housing isacquired without emitting light of approximately the first wavelengthfrom the one or more light emitters. At a second time later than thefirst time, a second measurement of light including the first wavelengthoriginating outside the housing is acquired while emitting light ofapproximately the first wavelength with at least one said light emitter.At a third time later than the second time, a third measurement of lightincluding the first wavelength originating outside the housing isacquired without emitting light of approximately the first wavelengthfrom the one or more light emitters. At a fourth time, a firstmeasurement of light including a second wavelength originating outsidethe housing is acquired without emitting light of approximately thesecond wavelength from the one or more light emitters. The secondwavelength is longer than the first wavelength. At a fifth time laterthan the fourth time, a second measurement of light including the secondwavelength originating outside the housing is acquired while emittinglight of approximately the second wavelength with at least one saidlight emitter. At a sixth time later than the fifth time, a thirdmeasurement of light including the second wavelength originating outsidethe housing is acquired without emitting light of approximately thesecond wavelength from the one or more light emitters. (As utilizedherein, light “originating outside the housing” includes portions oflight originally emitted by one or more of the light emitters andreflected back to one or more of the light detectors from an object or aplurality of particles in the external sampling area, as well as otherlight (e.g., background light) of the particular wavelength(s)originating from other sources and detected by one or more of the lightdetectors.) An ambient light level outside of the housing is detected.The second measurement of light including the first wavelength iscorrected based on (i) the detected ambient light level and (ii) thefirst and/or third measurements of light including the first wavelength,thereby producing a corrected first-wavelength measurement. The secondmeasurement of light including the second wavelength is corrected basedon (i) the detected ambient light level and (ii) the first and/or thirdmeasurements of light including the second wavelength, thereby producinga corrected second-wavelength measurement. The presence of smokeparticles outside the housing is determined based on a ratio of thecorrected first-wavelength measurement to the correctedsecond-wavelength measurement.

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. Measurements of lightincluding the first or second wavelength may be broadband measurementsof light of a broader range of wavelengths including the first or secondwavelengths (e.g., via a broadband detector), or they may be narrowbandmeasurements of light of a narrow band substantially equal to orincluding the first or second wavelength (e.g., via different narrowbanddetectors responsive only to particular wavelengths or wavelengthranges). Producing the corrected first-wavelength measurement mayinclude or consist essentially of (i) subtracting from the secondmeasurement of light including the first wavelength an average of thefirst and third measurements of light including the first wavelength and(ii) adding to the second measurement of light including the firstwavelength an offset based on a function of the detected ambient lightlevel. The offset may be based on a linear or polynomial function of thedetected ambient light level. Producing the corrected second-wavelengthmeasurement may include or consist essentially of (i) subtracting fromthe second measurement of light including the second wavelength anaverage of the first and third measurements of light including thesecond wavelength and (ii) adding to the second measurement of lightincluding the second wavelength an offset based on a function of thedetected ambient light level. The offset may be based on a linear orpolynomial function of the detected ambient light level. Determining thepresence of smoke particles outside the housing may include or consistessentially of comparing the ratio of the corrected first-wavelengthmeasurement to the corrected second-wavelength measurement to a firstthreshold, smoke particles being determined to be present when the ratioof the corrected first-wavelength measurement to the correctedsecond-wavelength measurement is larger than the first threshold. Thefirst threshold may correspond to a signal level larger than a signallevel generated via smoke obscuration outside the housing ofapproximately 0.5%/foot and/or to a signal level smaller than a signallevel generated via smoke obscuration outside the housing ofapproximately 4%/foot.

The presence of nuisance particles having a larger average diameter thanan average diameter of the smoke particles may be determined based onthe ratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement. Determining the presence of smokeparticles outside the housing may include or consist essentially ofcomparing the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement to a first threshold, smokeparticles being determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is larger than the first threshold, and determining thepresence of nuisance particles may include or consist essentially ofcomparing the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement to a second threshold, nuisanceparticles being determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is smaller than the second threshold. The first thresholdmay be approximately equal to the second threshold, or the secondthreshold may be lower than the first threshold.

The presence of an obstruction outside the housing may be determinedbased on the corrected first-wavelength measurement and/or the correctedsecond-wavelength measurement. Determining the presence of anobstruction may include or consist essentially of comparing thecorrected first-wavelength measurement and/or the correctedsecond-wavelength measurement to an obstruction threshold, anobstruction being determined to be present when the correctedfirst-wavelength measurement and/or the corrected second-wavelengthmeasurement is larger than the obstruction threshold. The obstructionthreshold may correspond to a signal level not achievable via buildup ofsmoke during a single measurement cycle of the one or more detectors.(In various embodiments, a single measurement cycle corresponds to thetime between measurements acquired by one or more detectors when in acontinuous or periodic monitoring mode.) The obstruction threshold maycorrespond to a signal level larger than a signal level generated viasmoke obscuration outside the housing of approximately 40%/foot.

Less than 100 milliseconds, or even less than 1 millisecond, may elapsebetween the first time and the third time. Less than 100 milliseconds,or even less than 1 millisecond, may elapse between the fourth time andthe sixth time. The one or more light emitters may include or consistessentially of a broadband light source emitting light over a range ofwavelengths, the first and second wavelengths being within the range ofwavelengths. The broadband light source may include or consistessentially of a white light-emitting diode. The one or more lightemitters may include or consist essentially of a first light emitteremitting light at the first wavelength and a second light emitter,different from the first light emitter, emitting light at the secondwavelength. The smoke detector may include a proximity sensor. At leastone light emitter and/or at least one light detector may be embeddedwithin the proximity sensor. The smoke detector may include an ambientlight sensor discrete from the proximity sensor. At least one lightdetector may be embedded within the ambient light sensor.

Light of the second wavelength may not be emitted at the second time.Light of the first wavelength may not be emitted at the fifth time. Thefirst wavelength may be between approximately 300 nm and approximately480 nm. The second wavelength may be between approximately 630 nm andapproximately 1000 nm. No light may be emitted by any of the lightemitters at the first and third times. No light may be emitted by any ofthe light emitters at the fourth and sixth times.

In another aspect, embodiments of the invention feature a smoke detectorincluding or consisting essentially of a housing, one or more lightemitters for emitting, outside the housing, light of a first wavelengthand a second wavelength longer than the first wavelength, one or morelight detectors for detecting (i) light emitted from the one or morelight emitters reflected back to the one or more light detectors,thereby providing measurements of reflected light including the firstand second wavelengths, and (ii) an ambient light level outside of thehousing, and an evaluation circuit for (i) correcting a measurement ofreflected light including the first wavelength based on the detectedambient light level, thereby producing a corrected first-wavelengthmeasurement, (ii) correcting a measurement of reflected light includingthe second wavelength based on the detected ambient light level, therebyproducing a corrected second-wavelength measurement, and (iii)determining the presence of smoke particles outside the housing based ona ratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement.

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. Measurements of lightincluding the first or second wavelength may be broadband measurementsof light of a broader range of wavelengths including the first or secondwavelengths (e.g., via a broadband detector), or they may be narrowbandmeasurements of light of a narrow band substantially equal to orincluding the first or second wavelength (e.g., via different narrowbanddetectors responsive only to particular wavelengths or wavelengthranges). The evaluation circuit may be configured to (via, e.g.,controlling components such as the one or more light emitters and theone or more light detectors) (i) at a first time, acquire a firstmeasurement of light including the first wavelength originating outsidethe housing without emitting light of approximately the first wavelengthfrom the one or more light emitters, (ii) at a second time later thanthe first time, acquire a second measurement of light including thefirst wavelength originating outside the housing while emitting light ofapproximately the first wavelength with at least one said light emitter,(iii) at a third time later than the second time, acquire a thirdmeasurement of light including the first wavelength originating outsidethe housing without emitting light of approximately the first wavelengthfrom the one or more light emitters, (iv) at a fourth time, acquire afirst measurement of light including the second wavelength originatingoutside the housing without emitting light of approximately the secondwavelength from the one or more light emitters, (v) at a fifth timelater than the fourth time, acquire a second measurement of lightincluding the second wavelength originating outside the housing whileemitting light of approximately the second wavelength with at least onesaid light emitter, (vi) at a sixth time later than the fifth time,acquire a third measurement of light including the second wavelengthoriginating outside the housing without emitting light of approximatelythe second wavelength from the one or more light emitters, and (vii)detect the ambient light level outside of the housing.

The evaluation circuit may be configured to produce the correctedfirst-wavelength measurement by (i) subtracting from the secondmeasurement of light including the first wavelength an average of thefirst and third measurements of light including the first wavelength and(ii) adding to the second measurement of light including the firstwavelength a first offset based on a function of the detected ambientlight level, and/or produce the corrected second-wavelength measurementby (i) subtracting from the second measurement of light including thesecond wavelength an average of the first and third measurements oflight including the second wavelength and (ii) adding to the secondmeasurement of light including the second wavelength a second offsetbased on a function of the detected ambient light level. The firstoffset may be based on a linear or polynomial function of the detectedambient light level. The second offset may be based on a linear orpolynomial function of the detected ambient light level. The evaluationcircuit may be configured to determine the presence of smoke particlesoutside the housing by comparing the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement to a first threshold, smoke particles being determined to bepresent when the ratio of the corrected first-wavelength measurement tothe corrected second-wavelength measurement is larger than the firstthreshold. The first threshold may correspond to a signal level largerthan a signal level generated via smoke obscuration outside the housingof approximately 0.5%/foot and/or to a signal level smaller than asignal level generated via smoke obscuration outside the housing ofapproximately 4%/foot.

The evaluation circuit may be configured to determine, based on theratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement, the presence of nuisance particles havinga larger average diameter than an average diameter of the smokeparticles. The evaluation circuit may be configured to (i) determine thepresence of smoke particles outside the housing by comparing the ratioof the corrected first-wavelength measurement to the correctedsecond-wavelength measurement to a first threshold, smoke particlesbeing determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is larger than the first threshold, and (ii) determine thepresence of nuisance particles by comparing the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement to a second threshold, nuisance particles being determinedto be present when the ratio of the corrected first-wavelengthmeasurement to the corrected second-wavelength measurement is smallerthan the second threshold. The first threshold may be approximatelyequal to the second threshold. The second threshold may be lower thanthe first threshold.

The evaluation circuit may be configured to determine the presence of anobstruction outside the housing based on the corrected first-wavelengthmeasurement and/or the corrected second-wavelength measurement. Theevaluation circuit may be configured to determine the presence of anobstruction by comparing the corrected first-wavelength measurementand/or the corrected second-wavelength measurement to an obstructionthreshold, an obstruction being determined to be present when thecorrected first-wavelength measurement and/or the correctedsecond-wavelength measurement is larger than the obstruction threshold.The obstruction threshold may correspond to a signal level notachievable via buildup of smoke during a single measurement cycle of theone or more detectors. The obstruction threshold may correspond to asignal level larger than a signal level generated via smoke obscurationoutside the housing of approximately 40%/foot.

At least one light emitter (or even all light emitters) may be disposedwithin the housing. The housing may define one or more openings throughwhich light from the light emitter(s) is emitted. The one or moreopenings may include or consist essentially of a plurality of differentopenings each associated with at least one light emitter. Each lightemitter may emit light through a different opening. The housing mayinclude or consist essentially of one or more solid windows throughwhich light from the light emitter(s) is emitted. The one or more solidwindows may include or consist essentially of a plurality of differentsolid windows each associated with at least one light emitter. Eachlight emitter may emit light through a different window. The one or morelight emitters may include or consist essentially of a broadband lightsource emitting light over a range of wavelengths, the first and secondwavelengths being within the range of wavelengths. The broadband lightsource may include or consist essentially of a white light-emittingdiode. The one or more light emitters may include or consist essentiallyof a first light emitter emitting light at the first wavelength and asecond light emitter, different from the first light emitter, emittinglight at the second wavelength. The first light emitter may beconfigured to emit light of the first wavelength only when the secondlight emitter is not emitting light of the second wavelength, and/or thesecond light emitter may be configured to emit light of the secondwavelength only when the first light emitter is not emitting light ofthe first wavelength. The smoke detector may include a proximity sensor.At least one light emitter and/or at least one light detector may beembedded within the proximity sensor. The one or more light detectorsmay include an ambient light sensor discrete from the proximity sensor.The proximity sensor may detect the ambient light level outside thehousing (i.e., the proximity sensor may include an ambient lightdetector therewithin). The first wavelength may be between approximately300 nm and approximately 480 nm. The second wavelength may be betweenapproximately 630 nm and approximately 1000 nm.

The evaluation circuit may include or consist essentially of a timer formeasuring elapsed time, a receiver for (i) receiving signals from atleast one light detector at a plurality of different times measured bythe timer and (ii) receiving signals based on the detected ambient lightlevel, a controller for controlling at least one light emitter to (i)emit light during at least one of the plurality of times during whichlight-detection signals are received and (ii) not emit light during atleast one other of the plurality of times during which light-detectionsignals are received, a transformer for producing the correctedfirst-wavelength measurement and the corrected second-wavelengthmeasurement based on signals received by the receiver, and a signalanalyzer for determining the presence of smoke particles outside thehousing. The controller may control a first light emitter emitting lightof the first wavelength and a second light emitter emitting light of thesecond wavelength.

The evaluation circuit may be configured to (a) at a first time, acquirea first measurement of light including the first wavelength originatingoutside the housing while emitting light of approximately the firstwavelength with at least one said light emitter, (b) at least one of (i)at a second time earlier than the first time, acquire a secondmeasurement of light including the first wavelength originating outsidethe housing without emitting light of approximately the first wavelengthfrom the one or more light emitters, or (ii) at a third time later thanthe first time, acquire a third measurement of light including the firstwavelength originating outside the housing without emitting light ofapproximately the first wavelength from the one or more light emitters,(c) at a fourth time, acquire a first measurement of light including thesecond wavelength originating outside the housing while emitting lightof approximately the second wavelength with at least one said lightemitter, (d) at least one of (i) at a fifth time earlier than the fourthtime, acquire a second measurement of light including the secondwavelength originating outside the housing without emitting light ofapproximately the second wavelength from the one or more light emitters,or (ii) at a sixth time later than the fourth time, acquire a thirdmeasurement of light including the second wavelength originating outsidethe housing without emitting light of approximately the secondwavelength from the one or more light emitters, and (e) detect anambient light level outside of the housing. The evaluation circuit maybe configured to (a) only acquire one of the second or thirdmeasurements of light including the first wavelength, and (b) producethe corrected first-wavelength measurement by (i) subtracting from thefirst measurement of light including the first wavelength the acquiredone of the second or third measurements of light including the firstwavelength and (ii) adding to the first measurement of light includingthe first wavelength an offset based on a function of the detectedambient light level. The evaluation circuit may be configured to (a)acquire both of the second and third measurements of light including thefirst wavelength, and (b) produce the corrected first-wavelengthmeasurement by (i) subtracting from the first measurement of lightincluding the first wavelength an average of the second and thirdmeasurements of light including the first wavelength and (ii) adding tothe first measurement of light including the first wavelength an offsetbased on a function of the detected ambient light level. The evaluationcircuit may be configured to (a) only acquire one of the second or thirdmeasurements of light including the second wavelength, and (b) producethe corrected second-wavelength measurement by (i) subtracting from thefirst measurement of light including the second wavelength the acquiredone of the second or third measurements of light including the secondwavelength and (ii) adding to the first measurement of light includingthe second wavelength an offset based on a function of the detectedambient light level. The evaluation circuit may be configured to (a)acquire both of the second and third measurements of light including thesecond wavelength, and (b) produce the corrected second-wavelengthmeasurement by (i) subtracting from the first measurement of lightincluding the second wavelength an average of the second and thirdmeasurements of light including the second wavelength and (ii) adding tothe first measurement of light including the second wavelength an offsetbased on a function of the detected ambient light level.

The evaluation circuit may be configured to (a) acquire a firstunilluminated measurement of light including the first wavelength andthe second wavelength originating outside the housing without emittinglight of approximately the first wavelength or light of approximatelythe second wavelength from the one or more light emitters, (b) acquire ameasurement of light including the first wavelength originating outsidethe housing while emitting light of approximately the first wavelengthwith at least one said light emitter, (c) acquire a measurement of lightincluding the second wavelength originating outside the housing whileemitting light of approximately the second wavelength with at least onesaid light emitter, and (d) detect an ambient light level outside of thehousing. The evaluation circuit may be configured to (a) produce thecorrected first-wavelength measurement by (i) subtracting from themeasurement of light including the first wavelength the firstunilluminated measurement of light including the first wavelength andthe second wavelength and (ii) adding to the measurement of lightincluding the first wavelength a first offset based on a function of thedetected ambient light level, and/or (b) produce the correctedsecond-wavelength measurement by (i) subtracting from the measurement oflight including the second wavelength the first unilluminatedmeasurement of light including the first wavelength and the secondwavelength and (ii) adding to the measurement of light including thesecond wavelength a second offset based on a function of the detectedambient light level. The first offset may be based on a linear orpolynomial function of the detected ambient light level. The secondoffset may be based on a linear or polynomial function of the detectedambient light level.

The evaluation circuit may be configured to acquire the firstunilluminated measurement of light including the first wavelength andthe second wavelength before the measurement of light including thefirst wavelength and the measurement of light including the secondwavelength are acquired. The evaluation circuit may be configured toacquire at least one of the measurement of light including the firstwavelength or the measurement of light including the second wavelengthbefore the first unilluminated measurement of light including the firstwavelength and the second wavelength is acquired. The evaluation circuitmay be configured to acquire only one of the measurement of lightincluding the first wavelength or the measurement of light including thesecond wavelength before the first unilluminated measurement of lightincluding the first wavelength and the second wavelength is acquired.

The evaluation circuit may be configured to, after acquiring the firstunilluminated measurement of light including the first wavelength andthe second wavelength, acquire a second unilluminated measurement oflight including the first wavelength and the second wavelengthoriginating outside the housing without emitting light of approximatelythe first wavelength or light of approximately the second wavelengthfrom the one or more light emitters. The evaluation circuit may beconfigured to (i) acquire the first unilluminated measurement of lightincluding the first wavelength and the second wavelength before at leastone of the measurement of light including the first wavelength or themeasurement of light including the second wavelength is acquired, and(ii) acquire the second unilluminated measurement of light including thefirst wavelength and the second wavelength after at least one of themeasurement of light including the first wavelength or the measurementof light including the second wavelength is acquired. The evaluationcircuit may be configured to produce the corrected first-wavelengthmeasurement by (i) subtracting from the measurement of light includingthe first wavelength an average of (a) the first unilluminatedmeasurement of light including the first wavelength and the secondwavelength and (b) the second unilluminated measurement of lightincluding the first wavelength and the second wavelength, and (ii)adding to the measurement of light including the first wavelength anoffset based on a function of the detected ambient light level. Theevaluation circuit may be configured to produce the correctedsecond-wavelength measurement by (i) subtracting from the measurement oflight including the second wavelength an average of (a) the firstunilluminated measurement of light including the first wavelength andthe second wavelength and (b) the second unilluminated measurement oflight including the first wavelength and the second wavelength, and (ii)adding to the measurement of light including the second wavelength anoffset based on a function of the detected ambient light level.

In yet another aspect, embodiments of the invention feature a method ofsmoke detection utilizing a smoke detector including or consistingessentially of (a) a housing, (b) one or more light emitters, and (c)one or more light detectors. At a first time, a first measurement oflight including a first wavelength originating outside the housing isacquired while emitting light of approximately the first wavelength withat least one said light emitter. At a second time earlier than the firsttime, a second measurement of light including the first wavelengthoriginating outside the housing is acquired without emitting light ofapproximately the first wavelength from the one or more light emitters,and/or, at a third time later than the first time, a third measurementof light including the first wavelength originating outside the housingis acquired without emitting light of approximately the first wavelengthfrom the one or more light emitters. At a fourth time, a firstmeasurement of light including a second wavelength originating outsidethe housing is acquired while emitting light of approximately the secondwavelength with at least one said light emitter. The second wavelengthis longer than the first wavelength. At a fifth time earlier than thefourth time, a second measurement of light including the secondwavelength originating outside the housing is acquired without emittinglight of approximately the second wavelength from the one or more lightemitters, and/or at a sixth time later than the fourth time, a thirdmeasurement of light including the second wavelength originating outsidethe housing is acquired without emitting light of approximately thesecond wavelength from the one or more light emitters. An ambient lightlevel outside of the housing is detected. The first measurement of lightincluding the first wavelength is corrected based on (i) the detectedambient light level and (ii) the second and/or third measurements oflight including the first wavelength, thereby producing a correctedfirst-wavelength measurement. The first measurement of light includingthe second wavelength is corrected based on (i) the detected ambientlight level and (ii) the second and/or third measurements of lightincluding the second wavelength, thereby producing a correctedsecond-wavelength measurement. The presence of smoke particles outsidethe housing is determined based on a ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement.

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. Measurements of lightincluding the first or second wavelength may be broadband measurementsof light of a broader range of wavelengths including the first or secondwavelengths (e.g., via a broadband detector), or they may be narrowbandmeasurements of light of a narrow band substantially equal to orincluding the first or second wavelength (e.g., via different narrowbanddetectors responsive only to particular wavelengths or wavelengthranges). Only one of the second or third measurements of light includingthe first wavelength may be acquired, and producing the correctedfirst-wavelength measurement may include or consist essentially of (i)subtracting from the first measurement of light including the firstwavelength the acquired one of the second or third measurements of lightincluding the first wavelength and (ii) adding to the first measurementof light including the first wavelength an offset based on a function ofthe detected ambient light level. The offset may be based on a linear orpolynomial function of the detected ambient light level. Both of thesecond and third measurements of light including the first wavelengthmay be acquired, and producing the corrected first-wavelengthmeasurement may include or consist essentially of (i) subtracting fromthe first measurement of light including the first wavelength an averageof the second and third measurements of light including the firstwavelength and (ii) adding to the first measurement of light includingthe first wavelength an offset based on a function of the detectedambient light level. The offset may be based on a linear or polynomialfunction of the detected ambient light level. Only one of the second orthird measurements of light including the second wavelength may beacquired, and producing the corrected second-wavelength measurement mayinclude or consist essentially of (i) subtracting from the firstmeasurement of light including the second wavelength the acquired one ofthe second or third measurements of light including the secondwavelength and (ii) adding to the first measurement of light includingthe second wavelength an offset based on a function of the detectedambient light level. The offset may be based on a linear or polynomialfunction of the detected ambient light level. Both of the second andthird measurements of light including the second wavelength may beacquired, and producing the corrected second-wavelength measurement mayinclude or consist essentially of (i) subtracting from the firstmeasurement of light including the second wavelength an average of thesecond and third measurements of light including the second wavelengthand (ii) adding to the first measurement of light including the secondwavelength an offset based on a function of the detected ambient lightlevel. The offset may be based on a linear or polynomial function of thedetected ambient light level.

Determining the presence of smoke particles outside the housing mayinclude or consist essentially of comparing the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement to a first threshold, smoke particles being determined to bepresent when the ratio of the corrected first-wavelength measurement tothe corrected second-wavelength measurement is larger than the firstthreshold. The first threshold may correspond to a signal level largerthan a signal level generated via smoke obscuration outside the housingof approximately 0.5%/foot and/or to a signal level smaller than asignal level generated via smoke obscuration outside the housing ofapproximately 4%/foot.

The presence of nuisance particles having a larger average diameter thanan average diameter of the smoke particles may be determined based onthe ratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement. Determining the presence of smokeparticles outside the housing may include or consist essentially ofcomparing the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement to a first threshold, smokeparticles being determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is larger than the first threshold, and determining thepresence of nuisance particles may include or consist essentially ofcomparing the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement to a second threshold, nuisanceparticles being determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is smaller than the second threshold. The first thresholdmay be approximately equal to the second threshold. The second thresholdmay be lower than the first threshold.

The presence of an obstruction outside the housing may be determinedbased on the corrected first-wavelength measurement and/or the correctedsecond-wavelength measurement. Determining the presence of anobstruction may include or consist essentially of comparing thecorrected first-wavelength measurement and/or the correctedsecond-wavelength measurement to an obstruction threshold, anobstruction being determined to be present when the correctedfirst-wavelength measurement and/or the corrected second-wavelengthmeasurement is larger than the obstruction threshold. The obstructionthreshold may correspond to a signal level not achievable via buildup ofsmoke during a single measurement cycle of the one or more detectors.The obstruction threshold may correspond to a signal level larger than asignal level generated via smoke obscuration outside the housing ofapproximately 40%/foot. The first measurement of light including thefirst wavelength and the second and/or third measurements of lightincluding the first wavelength may be acquired over a time period lessthan 100 milliseconds. The first measurement of light including thesecond wavelength and the second and/or third measurements of lightincluding the second wavelength may be acquired over a time period lessthan 100 milliseconds. The first measurement of light including thefirst wavelength and the second and/or third measurements of lightincluding the first wavelength may be acquired over a time period lessthan 1 millisecond. The first measurement of light including the secondwavelength and the second and/or third measurements of light includingthe second wavelength may be acquired over a time period less than 1millisecond.

The one or more light emitters may include or consist essentially of abroadband light source emitting light over a range of wavelengths, thefirst and second wavelengths being within the range of wavelengths. Thebroadband light source may include or consist essentially of a whitelight-emitting diode. The one or more light emitters may include orconsist essentially of a first light emitter emitting light at the firstwavelength and a second light emitter, different from the first lightemitter, emitting light at the second wavelength. The smoke detector mayinclude a proximity sensor. At least one light emitter and/or at leastone light detector may be portions of and/or embedded within theproximity sensor. The smoke detector may include an ambient light sensordiscrete from the proximity sensor. At least one light detector may bepart of and/or embedded within the ambient light sensor. Light of thesecond wavelength may not be emitted at the first time. Light of thefirst wavelength may not be emitted at the fourth time. The firstwavelength may be between approximately 300 nm and approximately 480 nm.The second wavelength may be between approximately 630 nm andapproximately 1000 nm. Light may not be emitted by any of the lightemitters at the second and third times. Light may not be emitted by anyof the light emitters at the fifth and sixth times.

In another aspect, embodiments of the invention feature a method ofsmoke detection utilizing a smoke detector comprising (a) a housing, (b)one or more light emitters, and (c) one or more light detectors. A firstunilluminated measurement of light including a first wavelength and asecond wavelength longer than the first wavelength originating outsidethe housing is acquired without emitting light of approximately thefirst wavelength or light of approximately the second wavelength fromthe one or more light emitters. A measurement of light including thefirst wavelength originating outside the housing is acquired whileemitting light of approximately the first wavelength with at least onesaid light emitter. A measurement of light including the secondwavelength originating outside the housing is acquired while emittinglight of approximately the second wavelength with at least one saidlight emitter. An ambient light level outside of the housing isdetected. The measurement of light including the first wavelength iscorrected based at least in part on (i) the detected ambient light leveland (ii) the first unilluminated measurement of light including thefirst wavelength and the second wavelength, thereby producing acorrected first-wavelength measurement. The measurement of lightincluding the second wavelength is corrected based at least in part on(i) the detected ambient light level and (ii) the first unilluminatedmeasurement of light including the first wavelength and the secondwavelength, thereby producing a corrected second-wavelength measurement.The presence of smoke particles outside the housing is determined basedon a ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement.

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. Measurements of lightincluding the first or second wavelength may be broadband measurementsof light of a broader range of wavelengths including the first or secondwavelengths (e.g., via a broadband detector), or they may be narrowbandmeasurements of light of a narrow band substantially equal to orincluding the first or second wavelength (e.g., via different narrowbanddetectors responsive only to particular wavelengths or wavelengthranges). Producing the corrected first-wavelength measurement mayinclude or consist essentially of (i) subtracting from the measurementof light including the first wavelength the first unilluminatedmeasurement of light including the first wavelength and the secondwavelength and (ii) adding to the measurement of light including thefirst wavelength an offset based on a function of the detected ambientlight level. The offset may be based on a linear or polynomial functionof the detected ambient light level. Producing the correctedsecond-wavelength measurement may include or consist essentially of (i)subtracting from the measurement of light including the secondwavelength the first unilluminated measurement of light including thefirst wavelength and the second wavelength and (ii) adding to themeasurement of light including the second wavelength an offset based ona function of the detected ambient light level. The offset may be basedon a linear or polynomial function of the detected ambient light level.

The first unilluminated measurement of light including the firstwavelength and the second wavelength may be acquired before themeasurement of light including the first wavelength and the measurementof light including the second wavelength are acquired. At least one ofthe measurement of light including the first wavelength or themeasurement of light including the second wavelength may be acquiredbefore the first unilluminated measurement of light including the firstwavelength and the second wavelength is acquired. Only one (i.e.,either) of the measurement of light including the first wavelength orthe measurement of light including the second wavelength may be acquiredbefore the first unilluminated measurement of light including the firstwavelength and the second wavelength is acquired.

After acquiring the first unilluminated measurement of light includingthe first wavelength and the second wavelength, a second unilluminatedmeasurement of light including the first wavelength and the secondwavelength originating outside the housing may be acquired withoutemitting light of approximately the first wavelength or light ofapproximately the second wavelength from the one or more light emitters.The first unilluminated measurement of light including the firstwavelength and the second wavelength may be acquired before at least oneof the measurement of light including the first wavelength or themeasurement of light including the second wavelength is acquired. Thesecond unilluminated measurement of light including the first wavelengthand the second wavelength may be acquired after at least one of themeasurement of light including the first wavelength or the measurementof light including the second wavelength is acquired. Producing thecorrected first-wavelength measurement may include or consistessentially of (i) subtracting from the measurement of light includingthe first wavelength an average of (a) the first unilluminatedmeasurement of light including the first wavelength and the secondwavelength and (b) the second unilluminated measurement of lightincluding the first wavelength and the second wavelength, and (ii)adding to the measurement of light including the first wavelength anoffset based on a function of the detected ambient light level. Theoffset may be based on a linear or polynomial function of the detectedambient light level. Producing the corrected second-wavelengthmeasurement may include or consist essentially of (i) subtracting fromthe measurement of light including the second wavelength an average of(a) the first unilluminated measurement of light including the firstwavelength and the second wavelength and (b) the second unilluminatedmeasurement of light including the first wavelength and the secondwavelength, and (ii) adding to the measurement of light including thesecond wavelength an offset based on a function of the detected ambientlight level. The offset may be based on a linear or polynomial functionof the detected ambient light level.

Determining the presence of smoke particles outside the housing mayinclude or consist essentially of comparing the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement to a first threshold, smoke particles being determined to bepresent when the ratio of the corrected first-wavelength measurement tothe corrected second-wavelength measurement is larger than the firstthreshold. The first threshold may correspond to a signal level largerthan a signal level generated via smoke obscuration outside the housingof approximately 0.5%/foot and/or to a signal level smaller than asignal level generated via smoke obscuration outside the housing ofapproximately 4%/foot.

The presence of nuisance particles having a larger average diameter thanan average diameter of the smoke particles may be determined based onthe ratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement. Determining the presence of smokeparticles outside the housing may include or consist essentially ofcomparing the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement to a first threshold, smokeparticles being determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is larger than the first threshold. Determining the presenceof nuisance particles may include or consist essentially of comparingthe ratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement to a second threshold, nuisance particlesbeing determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is smaller than the second threshold. The first thresholdmay be approximately equal to the second threshold. The second thresholdmay be lower than the first threshold.

The presence of an obstruction outside the housing may be determinedbased on at least one of the corrected first-wavelength measurement orthe corrected second-wavelength measurement. Determining the presence ofan obstruction may include or consist essentially of comparing thecorrected first-wavelength measurement and/or the correctedsecond-wavelength measurement to an obstruction threshold, anobstruction being determined to be present when the correctedfirst-wavelength measurement and/or the corrected second-wavelengthmeasurement is larger than the obstruction threshold. The obstructionthreshold may correspond to a signal level not achievable via buildup ofsmoke during a single measurement cycle of the one or more detectors.The obstruction threshold may correspond to a signal level larger than asignal level generated via smoke obscuration outside the housing ofapproximately 40%/foot.

The first unilluminated measurement of light including the firstwavelength and the second wavelength, the measurement of light includingthe first wavelength, and the measurement of light including the secondwavelength may be acquired over a time period less than 100milliseconds, or even a time period less than 1 millisecond. The one ormore light emitters may include or consist essentially of a broadbandlight source emitting light over a range of wavelengths, the first andsecond wavelengths being within the range of wavelengths. The broadbandlight source may include or consist essentially of a whitelight-emitting diode. The one or more light emitters may include orconsist essentially of a first light emitter emitting light at the firstwavelength and a second light emitter, different from the first lightemitter, emitting light at the second wavelength. The smoke detector mayinclude a proximity sensor, and at least one light detector may beembedded within the proximity sensor. The smoke detector may include anambient light sensor discrete from the proximity sensor, and at leastone light detector may be embedded within the ambient light sensor. Thefirst wavelength may be between approximately 300 nm and approximately480 nm (inclusive). The second wavelength may be between approximately630 nm and approximately 1000 nm (inclusive). Light may not be emittedby any of the one or more light emitters during acquisition of the firstunilluminated measurement of light including the first wavelength andthe second wavelength.

Embodiments of any of the above aspects of the invention may include oneor more of the following in any of a variety of combinations. One ormore light emitters may be configured to (i) emit a first light portionoutside of the housing and (ii) emit a second light portion within thehousing substantially without emission therefrom. One or more lightdetectors may be configured to receive light from the first lightportion reflected back into the housing and light from the second lightportion within the housing. The evaluation circuit may determine thepresence of smoke particles outside the housing based in part on (i) thelight received by the one or more light detectors from the first lightportion and (ii) the light received by the one or more light detectorsfrom the second light portion. The presence of smoke particles outsidethe housing may be determined based in part on (i) a luminance and/or arate of change of luminance of light received from the first lightportion, and (ii) a luminance of light received from the second lightportion. Light from the second light portion may be reflected by aportion of the housing proximate an opening in the housing. Light fromthe second light portion may be reflected by a window in the housing.One or more light emitters and/or one or more light detectors may beportions of a single electronic component, e.g., a proximity sensor. Oneor more light emitters and/or one or more light detectors may not beportions of a single electronic component (and may thus be separateelectronic components that are independently operable). A gas sensor maybe disposed at least partially within or on the housing. The presence ofsmoke particles outside the housing may be determined based in part on(i) a gas concentration sensed by the gas sensor and/or (ii) a temporalevolution of the gas concentration sensed by the gas sensor. The gassensor may be configured to sense carbon monoxide and/or carbon dioxide.A manual test button may be disposed on the housing and electricallyconnected to the evaluation circuit. After actuation of the manual testbutton, the evaluation circuit may perform a test sequence. The testsequence may be based at least in part on the luminance of the receivedfirst light portion and the luminance of the received second lightportion, and/or the corrected first-wavelength measurement and thecorrected second-wavelength measurement. The one or more light detectorsmay include or consist essentially of a plurality of light detectorseach sensitive to light over only a portion of a range of wavelengthsemitted by the one or more light emitters. The ambient-light sensor maysense visible and/or infrared light. Light reflected from the firstlight portion and light from the second light portion may be detected bythe same light detector. A maintenance alarm may be activated if aluminance of the detected light from the second light portion fallsbelow a maintenance threshold. A gas concentration outside the housing,e.g., a concentration of carbon monoxide and/or carbon dioxide, may besensed. The presence of smoke particles outside the housing may bedetermined based in part on the sensed gas concentration.

These and other objects, along with advantages and features of theinvention, will become more apparent through reference to the followingdescription, the accompanying drawings, and the claims. Furthermore, itis to be understood that the features of the various embodimentsdescribed herein are not mutually exclusive and can exist in variouscombinations and permutations. Reference throughout this specificationto “one example,” “an example,” “one embodiment,” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the example is included in at least one example ofthe present technology. Thus, the occurrences of the phrases “in oneexample,” “in an example,” “one embodiment,” or “an embodiment” invarious places throughout this specification are not necessarily allreferring to the same example. Furthermore, the particular features,structures, routines, steps, or characteristics may be combined in anysuitable manner in one or more examples of the technology. The term“light” broadly connotes any wavelength or wavelength band in theelectromagnetic spectrum, including, without limitation, visible light,ultraviolet radiation, and infrared radiation. Similarly, photometricterms such as “luminance,” “luminous flux,” and “luminous intensity”extend to and include their radiometric equivalents, such as “radiance,”“radiant flux,” and “radiant intensity.” As used herein, a “portion oflight” means an intensity or directional fraction of light that may ormay not be discrete from other portions of the same light. As usedherein, the term “substantially” means±10%, and in some embodiments,±5%. The term “consists essentially of” means excluding other materialsthat contribute to function, unless otherwise defined herein.Nonetheless, such other materials may be present, collectively orindividually, in trace amounts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a cross-sectional diagram of a smoke detector with discretelight emitters emitting at different wavelengths, a discrete proximitysensor, and a discrete ambient-light sensor in accordance with variousembodiments of the invention;

FIG. 1B is a cross-sectional diagram of a smoke detector with discretelight emitters emitting at different wavelengths and a proximity sensorfeaturing an embedded ambient-light sensor in accordance with variousembodiments of the invention;

FIG. 2A illustrates signal generation from smoke located in a samplingvolume and ambient light from an ambient light source in accordance withvarious embodiments of the invention;

FIG. 2B is a block diagram of an evaluation circuit in accordance withvarious embodiments of the invention;

FIG. 3 displays an ambient light signal, an uncorrected proximity sensorsignal, a partially corrected proximity sensor signal, and a correctedproximity sensor signal in accordance with various embodiments of theinvention;

FIG. 4 displays the ratio of corrected proximity sensor signals of twodifferent wavelengths for several nuisance and fire sources inaccordance with various embodiments of the invention; and

FIG. 5 is a flow chart depicting a method to distinguish smokeparticles, nuisance particles, and obstructions in accordance withvarious embodiments of the invention.

DETAILED DESCRIPTION

Discrimination between smoke particles and nuisance particles may beachieved by generating multiple signals each using distinct wavelengthsof light. Airborne particles other than smoke, such as dust, powders,cooking aerosols, or water vapor, scatter the various wavelengths oflight throughout the near ultraviolet, visible, and near infrared (e.g.,wavelengths of approximately 300-1000 nm) generally equally becausethese particles have a diameter on the order of several microns.However, smoke particles, which typically have a diameter of less thanone micron, typically scatter the shorter wavelengths of light much morestrongly than the longer wavelengths. By using multiple light emitters,at least one with a shorter emission wavelength, such as blue, violet,or ultraviolet (e.g., wavelengths of approximately 300-480 nm), and atleast one with a longer emission wavelength, such as red or infrared(e.g., wavelengths of approximately 630-1000 nm), the relative signalsmay be compared to determine whether the airborne particles within theexternal sampling volume are smoke particles or not. As known to thoseof skill in the art, light emitters such as light-emitting diodes (LEDs)and lasers that emit at particular wavelengths may be produced by, e.g.,selection and/or adjustment of the band gap and/or lasing cavity size ofa semiconductor-based light emitter.

FIG. 1A depicts a smoke detector in accordance with various embodimentsof the invention. As shown, the smoke detector includes a red lightemitter 500, a blue light emitter 502, a proximity sensor 106, and anambient-light sensor 400 that are mounted onto a circuit board 110 (orotherwise mounted within a surrounding housing 120). An evaluationcircuit 104 may also be mounted on the circuit board 110. All of thesecomponents are typically disposed inside a smoke-detector housing 120,which includes or consists essentially of one or more rigid materials(e.g., metal, plastic, etc.). In various embodiments of the invention,the housing 120 has a single opening 130 that is situated over the redlight emitter 500, the blue light emitter 502, the proximity sensor 106,and the ambient-light sensor 400. (As shown in FIG. 1A, the opening 130is “over” all of these components in the sense that it is disposedopposite the circuit board 110 on which these components are mounted; inembodiments in which the smoke detector is mounted, e.g., on a ceiling,the opening 130 would be disposed “under” or “beneath” all of thesecomponents as pictured.) A window may be disposed within (and at leastpartially close) the opening 130. The window may include or consistessentially of, e.g., plastic and/or glass, and is generally at leastpartially transparent to light emitted by the red light emitter 500,light emitted by the blue light emitter 502, and ambient light. Thehousing 120 may also have multiple openings, with each opening situatedover at least one component, and may have windows disposed within (andat least partially closing) one or more of the openings.

The red light emitter 500 and blue light emitter 502 emit atsubstantially different wavelengths. In various embodiments of thepresent invention, the red light emitter 500 emits red and/or infraredlight, and the blue light emitter 502 emits blue, violet, and/orultraviolet light. Generally, the blue light emitter 502 emits light ofa shorter wavelength than light emitted by red light emitter 500. Theblue light emitter 502 may emit light of a wavelength less thanapproximately 500 nm, and the red light emitter 500 may emit light of awavelength greater than approximately 500 nm. In various embodiments ofthe invention, more than two light emitters may be utilized in the smokedetector, each with a substantially different wavelength from the otherlight emitters. In various embodiments of the present invention, aseparate light detector may be utilized for each light emitter in thesmoke detector. In various embodiments of the present invention, a broadspectrum of light may be emitted from the smoke detector by a singlelight emitter, and multiple different light detectors, each with asensitivity to a different wavelength or range of wavelengths, may beutilized. For example, a first light detector may be more sensitive tored and/or infrared light, and a second light detector may be moresensitive to blue, violet, and/or ultraviolet light. In another example,the first light detector may be sensitive to both visible and infraredlight, and the second light detector may be sensitive to only visiblelight. The single broadband emitter typically emits light over a widerange of wavelengths, and may include or consist essentially of one ormore white LEDs (i.e., LEDs that emit white light or mixed light thatclosely approximates white light). Multiple different light emitterswith different emission wavelengths may also be used in conjunction withthe multiple light detectors. As known to those of skill in the art,light detectors such as photodetectors that are sensitive to light ofparticular wavelengths may be produced by, e.g., selection and/oradjustment of the band gap of a semiconductor-based light detector

At least one light detector may be part of and may be embedded in theproximity sensor 106. The proximity sensor 106 may also control theoperation of the red light emitter 500 and blue light emitter 502, whichmay be components separate and discrete from proximity sensor 106. Anexemplary proximity sensor 106 in this embodiment is the SiliconLaboratories Si114x Proximity/Ambient Light Sensor, available fromSilicon Laboratories Inc. of Austin, Tex. At least one of the red lightemitter 500 and blue light emitter 502 may also be embedded in theproximity sensor 106. An exemplary proximity sensor 106 in thisembodiment is the Vishay Intertechnology VCNL4000 Fully IntegratedProximity and Ambient Light Sensor, available from VishayIntertechnology, Inc. of Malvern, Pa. If not embedded in the proximitysensor 106, the red light emitter 500 and blue light emitter 502 may beexternally driven by the proximity sensor 106. At least one lightdetector is a part of and may be embedded in the ambient-light sensor400. The light detector in the ambient-light sensor 400 is generallysensitive to visible light, but it may also be sensitive to ultravioletand/or infrared light. A light detector includes or consists essentiallyof one or more devices that register the presence of and/or measure aproperty the light illuminating the device(s). For example, the lightdetector may produce charge (i.e., an electronic signal) when exposed tolight. Exemplary light detectors include photodiodes, photodetectors,photoconductors, and/or photocapacitors. Alternative embodiments of theinvention use a discrete light emitter and light detector in place ofthe proximity sensor without altering the functionality of the smokedetector. Other alternative embodiments of the invention use a discretelight detector in place of the ambient-light sensor without altering thefunctionality of the smoke detector.

As shown in FIG. 1B, in a preferred embodiment of the present invention,the ambient-light sensor 400 is part of and even embedded within theproximity sensor 106 to form an integrated proximity/ambient-lightsensor 140. In the integrated proximity/ambient-light sensor 140, theproximity sensor control circuitry is typically separate from theambient-light sensor control circuitry, but the proximity sensor andambient-light sensor may use at least one common light detector. Atleast one of the red light emitter 500 and blue light emitter 502 mayalso be part of and even embedded in the integratedproximity/ambient-light sensor 140.

Electronic signals are generated when light is collected (or “sensed” or“detected”) by the light detectors embedded in the proximity sensor 106,integrated proximity/ambient-light sensor 140, and ambient-light sensor400. As shown in FIG. 2A, at least three signals may be generated whenairborne particles 530 are present in the external sampling volume. Anemitted beam 520 (which may include or consist essentially of, e.g., redor infrared light) from red light emitter 500 may pass through theopening 130 in the housing 120 and be scattered by airborne particles530, generating a red scattered beam 522. At least some of the redscattered beam 522 may pass back through the opening 130 in housing 120and be collected by the proximity sensor 106, producing a “red signal.”An emitted beam 524 (which may include or consist essentially of, e.g.,blue, violet, and/or ultraviolet light) from blue light emitter 502 mayalso pass through the opening 130 in housing 120 and be scattered by theairborne particles 530. At least some of the blue scattered beam 526 maypass back through the opening 130 in housing 120 and be collected by theproximity sensor 106, generating a “blue signal.” An ambient light beam214 from an ambient light source 212 may also pass through the opening130 in housing 120 and be collected by the ambient-light sensor 400,generating an “ambient signal.” Example ambient light sources includesunlight or light from incandescent, fluorescent, halogen, or LED lightbulbs. The ambient light beam 214, after passing through the opening 130in housing 120, may also be partially collected by the proximity sensor106, and may thus contribute to the red signal and/or the blue signal.

Signals may also be generated when an obstruction is present in theexternal sampling volume. The obstruction may be any object other thansmoke particles or nuisance particles, such as but not limited to aperson, furniture, or a cleaning instrument.

In various embodiments of the invention, the light emitted by the redlight emitter 500 and blue light emitter 502 may be separately pulsed totemporally distinguish the signals from each other and to reduce powerconsumption. For example, only one of the light emitters 500, 502 may beemitting light at any particular time. As another example, the bluelight emitter 502 may be pulsed less frequently than the red lightemitter 500 to be more visually inconspicuous to a person near the smokedetector. Thus, the blue signal may be collected and/or processed lessfrequently than the red signal.

At least portions of the signals collected by the light detectors in theproximity sensor 106 and ambient-light sensor 400 are typicallytransmitted to the evaluation circuit 104, which analyzes the signals todetermine whether smoke particles, nuisance particles, or an obstructionis present in the sampling volume. FIG. 2B schematically depicts variouscomponents of the evaluation circuit 104, which may include (but not belimited to) a memory 240, a receiver 250, a signal analyzer 260, atransformer 270, a controller 280, and/or a timer 290. The memory 240may store pre-determined values (e.g., thresholds) utilized in sensingand/or control operations, and/or may store various signal values duringand/or after they are sensed, corrected, and/or transformed (e.g.,smoothed). At least a portion of memory 240 may be volatile, and atleast a portion of memory 240 may be non-volatile. The receiver 250 mayreceive signals from other components of the smoke detector (e.g., lightdetectors and other sensors) and route the signals to other portions ofthe evaluation circuit 104. The signal analyzer 260 may compare received(and/or corrected and/or transformed) signals to various pre-determinedthreshold levels and/or to previously received (and/or corrected and/ortransformed) signals to determine if smoke particles, nuisanceparticles, or an obstruction is present. The transformer (or “transformmodule” or “transformation module”) 270 may transform received signalsto, e.g., reduce or eliminate noise and/or compensate for drift. Forexample, the transformer 270 may implement smoothing (e.g., exponentialsmoothing and/or moving-average smoothing), filtering (e.g., high-pass,low-pass, and/or band-pass filtering), regression, and/or othernumerical transformation techniques. The transformer 270 may alsocorrect received signals based on, e.g., other received signals from oneor more light detectors and/or ambient-light sensors, as detailed below.The controller 280 may control light emitters, light detectors, and/orother components of the smoke detector; for example, the controller 280may control speakers that emit audible alarms and/or light sources inresponse to a sensed alarm condition or as part of a test sequence. Thetimer 290 may measure time elapsed during or since various sensedconditions and/or may be utilized to measure pre-determined delaysutilized in various sensing or testing sequences.

The evaluation circuit 104 (and/or any or all of its components) may bea general-purpose microprocessor, but depending on implementation mayalternatively be a microcontroller, peripheral integrated circuitelement, a customer-specific integrated circuit (CSIC), anapplication-specific integrated circuit (ASIC), a logic circuit, adigital signal processor, a programmable logic device such as afield-programmable gate array (FPGA), a programmable logic device (PLD),a programmable logic array (PLA), an RFID processor, smart chip, or anyother device or arrangement of devices that is capable of implementingthe steps of the processes of embodiments of the invention. In apreferred embodiment, the evaluation circuit 104 is a microcontroller.The evaluation circuit 104 may be monolithically integrated with, andthus a portion of the same integrated-circuit chip as the proximitysensor 106 and/or ambient-light sensor 400, or evaluation circuit 104may be disposed on a chip separate and discrete from the chip containingthe proximity sensor 106 and/or ambient-light sensor 400 (andinterconnected thereto by wired or wireless means). Moreover, at leastsome of the functions of evaluation circuit 104 may be implemented insoftware and/or as mixed hardware-software modules. Software programsimplementing the functionality herein described may be written in any ofa number of high level languages such as FORTRAN, PASCAL, JAVA, C, C++,C#, BASIC, various scripting languages, and/or HTML. Additionally, thesoftware may be implemented in an assembly language directed to amicroprocessor resident in evaluation circuit 104. The software may beembodied on an article of manufacture including, but not limited to, afloppy disk, a jump drive, a hard disk, an optical disk, a magnetictape, a PROM, an EPROM, EEPROM, field-programmable gate array, CDROM, orDVDROM. Embodiments using hardware-software modules may be implementedusing, for example, one or more FPGA, CPLD, or ASIC processors.

As mentioned above, the luminance of the ambient light beam 214 maypartially contribute to the red signal and/or blue signal measured bythe proximity sensor. When there is a change in the ambient light level,this may cause a change in the red and/or blue signal, which may cause afalse alarm even though there are no particles or objects in thesampling volume. The change in ambient light level may occur nearlyinstantaneously, such when a room light is turned on or there is ACripple in the luminance output of a light bulb, or the change in ambientlight level may occur much more slowly, such as near dusk or dawn whenthe sun rises or sets.

In preferred embodiments of the present invention, to compensate forchanges in the ambient light level, the red signal and blue signalmeasured by the proximity sensor 106 are corrected by the evaluationcircuit 104 based on the value of the ambient signal measured by theambient-light sensor 400. FIG. 3 illustrates the application of ambientlight correction in accordance with various embodiments of theinvention. In the exemplary signals in FIG. 3, the room is initiallydark (as perceived by the naked eye), then at a first time anincandescent light bulb is turned on, then at a second time theincandescent light bulb is turned off. At no time do particles orobstructions enter the sampling volume. In an exemplary temporal ambientlight signal 310, three distinct regions are observed: a first regionwhen the room is dark and the ambient light level is small and static, asecond region when the incandescent light bulb is turned on and theambient light level is elevated and dynamic, and a third region when theincandescent light bulb is turned off and the ambient light level isonce again small and static. The sawtooth pattern in the second regionis due to the AC ripple in the luminance output of the incandescentlight bulb, which is absent when the light bulb is off. An exemplarytemporal uncorrected red signal 320 is output by the proximity sensor106 over the same time interval as the ambient light signal 310. In thefirst and third regions, the uncorrected red signal 320 is approximatelystatic. In the second region, the uncorrected red signal 320 has asimilar sawtooth pattern that temporally correlates with the ambientlight signal 310. In various embodiments of the invention, theuncorrected red signal 320 undergoes a first correction to eliminate thesawtooth pattern. In the first correction, the proximity sensor 106takes three measurements within a short time period, e.g., less than onemillisecond: a first measurement when the red light emitter 500 isunilluminated, a second measurement when the red light emitter 500 isilluminated, and a third measurement when the red light emitter 500 isagain unilluminated. The first and third measurements are averaged(e.g., an unweighted average or a weighted average weighting one of themeasurements more than the other) and subtracted from the secondmeasurement. In an alternative embodiment, measurements are taken atleast as frequently as the Nyquist rate of the AC ripple and theamplitude of the ripple is fitted by the evaluation circuit 104 andsubstantially eliminated from the uncorrected red signal 320. The resultafter the first correction is a partially corrected red signal 322. Byperforming the three measurements over a time period (e.g., <1 ms) muchshorter than the 120 Hz AC light ripple time period (˜8.3 ms), thesawtooth pattern in the partially corrected red signal 322 iseliminated; however, there is still a residual offset in the secondregion. The magnitude of the offset is related to the luminance of theambient light level. In some embodiments of the present invention, onlyone measurement when the red light emitter 500 is unilluminated isperformed (either before or after the measurement when the red lightemitter is illuminated), and/or utilized (i.e., without averaging) tocorrect the illuminated signal to produce the partially corrected redsignal 322; however, embodiments utilizing multiple measurements whenthe light emitter is unilluminated may provide signals from which thepresence of smoke and/or nuisance particles may be more accuratelydetermined, particularly in environments with rapidly changing lightlevels and/or when light emitters emit noisy or highly oscillatorylight. In various embodiments of the invention, the partially correlatedred signal 322 undergoes a second correction based on the ambient signalmeasured by ambient-light sensor 400. In the second correction, thepartially corrected red signal is adjusted by the ambient signalaccording to:R _(c) =R+ƒ(A,R),where R_(c) is the corrected red signal, R is the uncorrected (orpartially corrected) red signal, and A is the ambient signal. Thefunction ƒ(A,R) may be a linear or polynomial function of A only, Ronly, or both A and R. In a preferred embodiment, the function ƒ(A,R) isa linear function of A only taking the form ƒ(A)=mA, where m is aconstant scalar. The result after the second correction is a correctedred signal 324. The second correction decreases or substantiallyeliminates the residual offset in the second region. Although only thecorrection of the red signal was illustrated in this experiment, boththe red signal and blue signal may be corrected using this technique. Insome embodiments, the red and blue signal are both corrected based onthe same one or more unilluminated measurements (i.e., measurementstaken without emission of red or blue light).

If the luminance of the ambient light beam 214 onto the proximity sensor106 and/or the ambient-light sensor 400 becomes very intense, such aswhen the sensors are directly illuminated by the sun or a very brightlight bulb, either sensor may saturate, which prevents them fromoutputting signals and may even effectively halt the operation of thesmoke detector. If either sensor becomes saturated or reaches athreshold signal near the saturation level (e.g., 90% of the saturationlevel), the evaluation circuit 104 may switch to a different lightdetector embedded in the proximity sensor 106 or ambient-light sensor400 with a lower responsivity to avoid the saturation condition andensure operation of the smoke detector even when directly exposed tovery high ambient light levels. Alternatively, if either sensor becomessaturated or reaches a threshold signal near the saturation level, theevaluation circuit 104 may lower the gain of the light detector embeddedin the proximity sensor 106 or ambient-light sensor 400 to avoid thesaturation condition.

The corrected red and blue signals may be used to determine if particlesinside the sampling volume are smoke particles or nuisance particles. Asmentioned above, nuisance particles scatter red (and infrared) and blue(and violet and ultraviolet) light generally equally because theseparticles have diameters on the order of several microns, whereas smokeparticles scatter blue light more strongly than red light because theseparticles have diameters of less than one micron. By taking the ratio orthe difference between the corrected blue signal and corrected redsignal of particles in the sampling volume, the evaluation circuit 104may determine if the particles are smoke particles or nuisanceparticles. FIG. 4 shows the ratio of the corrected blue signal to thecorrected red signal (designated B_(c)/R_(c)) of various sources inaccordance with various embodiments of the present invention. Inexperimental trials, both nuisance sources (represented by white bars)and fire sources (represented by black bars) were tested. Fire sourcesare divided into smoldering fires (represented by the letter ‘S’) andflaming fires (represented by the letter ‘F’). The corrected blue/redsignal ratio of the nuisance sources is generally smaller than thecorrected blue/red signal ratio of the fire sources. Also, the correctedblue/red signal ratio of the smoldering fire sources is generallysmaller than the corrected blue/red signal ratio of the flaming firesources. In an embodiment of the invention, if the corrected blue/redsignal ratio is above a first ratio threshold 420, then the particles inthe sampling volume are determined to be smoke particles. If thecorrected blue/red signal ratio is below a second ratio threshold 422,then the particles in the sampling volume are determined to be nuisanceparticles. If the corrected blue/red signal ratio is in between thefirst ratio threshold 420 and second ratio threshold 422, then nodetermination on the particles is made and additional measurements maybe taken. In some embodiments of the invention, the first and secondratio thresholds 420, 422 are substantially equal—i.e., only one ratiothreshold is utilized to determine between smoke and nuisance particles.

To minimize the effects of noise and drift in a detected signal (e.g.,the red signal, blue signal, or ambient signal), the evaluation circuit104 may apply smoothing to the signal. In a preferred embodiment, thesmoothing is an exponential smoothing. Specifically, for a currentsensor reading x, the smoothed signal S is assigned the following value:S:=αx+(1−α)S,where α is the smoothing factor. As implied by the use of the assignmentoperator (‘:=’) in the above expression, the smoothed signal S may beupdated without the use of another variable. The smoothing factor α isin the range of 0<α<1.

In various embodiments of the present invention, slowly varying andquickly varying signals may be distinguished by calculating two smoothedsignals and taking the difference. The first smoothed signal has alarger smoothing factor α, typically in the range of 0.01<α<1. It maytrack signals that change over the course of seconds or minutes withoutsignificant lag. The second smoothed signal has a smaller smoothingfactor α, typically in the range of 0.0001<α<0.01. It may only tracksignals that change over the course of hours without significant lag.When there is a slowly varying drift in the signal, both the first andsecond smoothed signals may track the drift without significant lag. Thedifferential signal in this case will typically be approximately zero.In contrast, the insertion of smoke particles, nuisance particles, or anobstruction in the sample volume results in a more quickly varyingchange in the signal. The first smoothed signal may track the changewithout significant lag but the second smoothed signal generally willnot. The differential signal in this case will typically have a positivevalue that may exceed an alarm threshold value.

If the second smoothed signal is ever larger than the first smoothedsignal, which may occur if there is a decrease in the detected signal,then the second smoothed signal is assigned the value of the firstsmoothed signal. This ensures the differential signal will always bepositive when there is an increase in the detected signal, so that anypotential alarm condition will not be delayed or undetected.

The differential signal, based on either the corrected red signal orcorrected blue signal (and hereafter referred to as the signal), may beused to determine if an object inside the sampling volume is particlesor an obstruction. This may be accomplished by establishing twothresholds, an obstruction threshold and a smoke threshold. A solidobject has a much larger cross-sectional area than smoke particles;therefore, the object will generally produce a distinctly strongersignal than the smoke particles, even for very high smoke obscurations(or densities) of greater than 40%/ft. Thus, the obstruction thresholdis preferably set higher than the signal generated when the smokeobscuration is approximately 40%/ft. If the signal exceeds theobstruction threshold for a pre-determined amount of time, anobstruction alarm (i.e., an audible tone or visible light on the smokedetector itself or on an external notification device) may be activated.The pre-determined delay eliminates unwanted (or “false”) alarms fromfleeting events such as an insect passing through the external samplingvolume.

The smoke threshold is typically set lower than the obstructionthreshold. The smoke threshold may correspond to the signal generatedwhen the smoke obscuration exceeds approximately 0.5%/ft but typicallynot greater than approximately 4%/ft in the external sampling volume. Ifthe signal exceeds the smoke threshold for a pre-determined amount oftime, a smoke alarm (i.e., an audible tone or visible light on the smokedetector itself or on an external notification device) may be activated.The smoke alarm may be different from the obstruction alarm in tone,duration, volume, intensity, color, and/or frequency.

Manual system testing of the smoke detector may be performed byinserting an object, such as a hand or broom handle, into the externalsampling volume for a pre-determined amount of time (e.g., a minimumduration of 2-20 seconds) to intentionally increase the signal andactivate either the obstruction alarm or smoke alarm. If an alarm isalready activated, an object may be inserted into the external samplingvolume for a pre-determined amount of time to temporarily or permanently(at least for the currently sensed condition and/or until the smokedetector is reset) silence the alarm.

In various embodiments above, the evaluation circuit 104 analyzes thetemporal pattern of detected signal(s) to determine whether there aresmoke particles, nuisance particles, or an obstruction present in thesampling volume of the smoke detector, and take the appropriate actionof whether to activate a smoke alarm, obstruction alarm, or no alarm.Another exemplary technique of determining which condition is present,if any, is illustrated in FIG. 5. This is an exemplary standby sequencethat may be followed by a smoke detector when no detected or calculatedsignals have exceeded any threshold values and that may be executed byevaluation circuit 104 (e.g., as depicted in FIG. 2B).

In a process step 540, the uncorrected red signal R, uncorrected bluesignal B, and ambient signal A are measured. In an embodiment, any ofthe signals may be the average of multiple measurements. In anotherembodiment, signal smoothing may be applied to any or all of thesesignals. In another embodiment, the measurements of all signals occur inless than 100 milliseconds, and preferably in less than 1 millisecond.In another embodiment, a short delay (for example of approximately0.1-10 seconds) may be inserted before the measurement to reduce powerconsumption of the smoke detector. Such reduction in power consumptionmay be important when the smoke detector is powered by a battery toincrease battery lifetime. In another embodiment, the uncorrected bluesignal may be measured less frequently than the uncorrected red signal.

In a process step 542, the corrected red signal R_(c) and corrected bluesignal B_(c) are calculated based on the uncorrected red signal R,uncorrected blue signal B, and ambient signal A. In an embodiment,R_(c)=R+ƒ(A,R) and B_(c)=B+ƒ(A,B), as described above. In a preferredembodiment, R_(c)=R+mA and B_(c)=B+nA, where m and n are scalarconstants.

In a decision step 544, if R_(c) is greater than a specified obstructionthreshold, then an obstruction is present in the sampling volume and anobstruction detection sequence 552 may be activated. When smokeparticles or nuisance particles are present in the external samplingvolume, even with a very high obscuration density, the amount ofscattered light from the particles is typically still less than theamount of reflected light from a physical obstruction in the externalsampling volume. This is particularly true because the particlestypically would not build up to a high obscuration density within onemeasurement cycle of the standby sequence, whereas a physicalobstruction may be inserted into the external sampling volume within onemeasurement cycle, leading to a large increase in R_(c) between cycles.The obstruction threshold is preferably set at a level that cannotreasonably be reached by the buildup of smoke within one measurementcycle. In an alternative embodiment, B_(c) is used as the determiningvariable instead of R_(c).

In a decision step 546, if R_(c) is greater than a specified smokethreshold (and less than the obstruction threshold), then it indicatesthat something other than an obstruction is present in the samplingvolume. If the condition is not true, then process step 540 is repeated.The smoke threshold is typically less than the obstruction threshold. Aswith almost any electrical signal, the signal will typically containnoise, which may be characterized as a random signal added to the “true”signal. The smoke threshold is preferably set at a level that cannotreasonably be reached through the addition of noise. In a preferredembodiment, the smoke threshold may correspond to the signal generatedwhen the smoke obscuration exceeds approximately 0.5%/ft but typicallynot greater than approximately 4%/ft in the external sampling volume. Inan alternative embodiment, B_(c) is used as the determining variableinstead of R_(c).

In a decision step 548, if the ratio B_(c)/R_(c) is greater than aspecified first ratio threshold (e.g., first ratio threshold 420), thensmoke particles are present in the sampling volume and a smoke detectionsequence 554 may be activated. The first ratio threshold is typicallyless than the ratios measured for smoke particles generated by flamingand smoldering fires. The first ratio threshold is typically greaterthan the second ratio threshold described below.

In a decision step 550, if the ratio B_(c)/R_(c) is less than aspecified second ratio threshold (e.g., second ratio threshold 422),then nuisance particles are present in the sampling volume and anuisance detection sequence 556 may be activated. If the condition isnot true, then process step 540 is repeated. The second ratio thresholdis typically less than the first ratio threshold. The second ratiothreshold is typically greater than the ratios measured for nuisanceparticles. In an embodiment, the first ratio threshold and second ratiothreshold may be approximately equal.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A smoke detector comprising: a housing; one ormore light emitters for emitting, outside the housing, light of a firstwavelength and a second wavelength longer than the first wavelength; oneor more light detectors for detecting (i) light emitted from the one ormore light emitters reflected back to the one or more light detectors,thereby providing measurements of reflected light including the firstand second wavelengths, and (ii) an ambient light level outside of thehousing; and an evaluation circuit for (i) correcting a measurement ofreflected light including the first wavelength based on the detectedambient light level, thereby producing a corrected first-wavelengthmeasurement, (ii) correcting a measurement of reflected light includingthe second wavelength based on the detected ambient light level, therebyproducing a corrected second-wavelength measurement, and (iii)determining the presence of smoke particles outside the housing based ona ratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement.
 2. The smoke detector of claim 1, whereinthe evaluation circuit is configured to: at a first time, acquire afirst measurement of light including the first wavelength originatingoutside the housing without emitting light of approximately the firstwavelength from the one or more light emitters; at a second time laterthan the first time, acquire a second measurement of light including thefirst wavelength originating outside the housing while emitting light ofapproximately the first wavelength with at least one said light emitter;at a third time later than the second time, acquire a third measurementof light including the first wavelength originating outside the housingwithout emitting light of approximately the first wavelength from theone or more light emitters; at a fourth time, acquire a firstmeasurement of light including the second wavelength originating outsidethe housing without emitting light of approximately the secondwavelength from the one or more light emitters; at a fifth time laterthan the fourth time, acquire a second measurement of light includingthe second wavelength originating outside the housing while emittinglight of approximately the second wavelength with at least one saidlight emitter; at a sixth time later than the fifth time, acquire athird measurement of light including the second wavelength originatingoutside the housing without emitting light of approximately the secondwavelength from the one or more light emitters; and detect the ambientlight level outside of the housing.
 3. The smoke detector of claim 2,wherein the evaluation circuit is configured to: produce the correctedfirst-wavelength measurement by (i) subtracting from the secondmeasurement of light including the first wavelength an average of thefirst and third measurements of light including the first wavelength and(ii) adding to the second measurement of light including the firstwavelength a first offset based on a function of the detected ambientlight level; and produce the corrected second-wavelength measurement by(i) subtracting from the second measurement of light including thesecond wavelength an average of the first and third measurements oflight including the second wavelength and (ii) adding to the secondmeasurement of light including the second wavelength a second offsetbased on a function of the detected ambient light level.
 4. The smokedetector of claim 1, wherein the evaluation circuit is configured todetermine the presence of smoke particles outside the housing bycomparing the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement to a first threshold, smokeparticles being determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is larger than the first threshold.
 5. The smoke detector ofclaim 4, wherein the first threshold corresponds to at least one of (i)a signal level larger than a signal level generated via smokeobscuration outside the housing of approximately 0.5%/foot or (ii) asignal level smaller than a signal level generated via smoke obscurationoutside the housing of approximately 4%/foot.
 6. The smoke detector ofclaim 1, wherein: the evaluation circuit is configured to determine,based on the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement, the presence of nuisanceparticles having a larger average diameter than an average diameter ofthe smoke particles, the evaluation circuit is configured to determinethe presence of smoke particles outside the housing by comparing theratio of the corrected first-wavelength measurement to the correctedsecond-wavelength measurement to a first threshold, smoke particlesbeing determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is larger than the first threshold, and the evaluationcircuit is configured to determine the presence of nuisance particles bycomparing the ratio of the corrected first-wavelength measurement to thecorrected second-wavelength measurement to a second threshold, nuisanceparticles being determined to be present when the ratio of the correctedfirst-wavelength measurement to the corrected second-wavelengthmeasurement is smaller than the second threshold.
 7. The smoke detectorof claim 6, wherein the first threshold is approximately equal to thesecond threshold.
 8. The smoke detector of claim 6, wherein the secondthreshold is lower than the first threshold.
 9. The smoke detector ofclaim 1, wherein the evaluation circuit is configured to determine thepresence of an obstruction outside the housing by comparing at least oneof the corrected first-wavelength measurement or the correctedsecond-wavelength measurement to an obstruction threshold, anobstruction being determined to be present when at least one of thecorrected first-wavelength measurement or the correctedsecond-wavelength measurement is larger than the obstruction threshold.10. The smoke detector of claim 1, wherein at least one said lightemitter is disposed within the housing.
 11. The smoke detector of claim10, wherein the housing defines one or more openings through which lightfrom the at least one said light emitter is emitted.
 12. The smokedetector of claim 11, wherein the one or more openings comprise aplurality of different openings each associated with at least one lightemitter.
 13. The smoke detector of claim 10, wherein the housingcomprises one or more solid windows through which light from the atleast one said light emitter is emitted.
 14. The smoke detector of claim13, wherein the one or more solid windows comprise a plurality ofdifferent solid windows each associated with at least one light emitter.15. The smoke detector of claim 1, wherein the one or more lightemitters comprises a first light emitter emitting light at the firstwavelength and a second light emitter, different from the first lightemitter, emitting light at the second wavelength.
 16. The smoke detectorof claim 15, wherein (i) the first light emitter is configured to emitlight of the first wavelength only when the second light emitter is notemitting light of the second wavelength, and (ii) the second lightemitter is configured to emit light of the second wavelength only whenthe first light emitter is not emitting light of the first wavelength.17. The smoke detector of claim 1, further comprising a proximitysensor, at least one said light detector being embedded within theproximity sensor.
 18. The smoke detector of claim 17, wherein the one ormore light detectors comprise an ambient light sensor discrete from theproximity sensor.
 19. The smoke detector of claim 17, wherein theproximity sensor detects the ambient light level outside the housing.20. The smoke detector of claim 1, wherein the first wavelength isbetween approximately 300 nm and approximately 480 nm.
 21. The smokedetector of claim 1, wherein the second wavelength is betweenapproximately 630 nm and approximately 1000 nm.
 22. The smoke detectorof claim 1, wherein the evaluation circuit is configured to: at a firsttime, acquire a first measurement of light including the firstwavelength originating outside the housing while emitting light ofapproximately the first wavelength with at least one said light emitter;at least one of (i) at a second time earlier than the first time,acquire a second measurement of light including the first wavelengthoriginating outside the housing without emitting light of approximatelythe first wavelength from the one or more light emitters, or (ii) at athird time later than the first time, acquire a third measurement oflight including the first wavelength originating outside the housingwithout emitting light of approximately the first wavelength from theone or more light emitters; at a fourth time, acquire a firstmeasurement of light including the second wavelength originating outsidethe housing while emitting light of approximately the second wavelengthwith at least one said light emitter; at least one of (i) at a fifthtime earlier than the fourth time, acquire a second measurement of lightincluding the second wavelength originating outside the housing withoutemitting light of approximately the second wavelength from the one ormore light emitters, or (ii) at a sixth time later than the fourth time,acquire a third measurement of light including the second wavelengthoriginating outside the housing without emitting light of approximatelythe second wavelength from the one or more light emitters; and detect anambient light level outside of the housing.
 23. The smoke detector ofclaim 1, wherein the evaluation circuit comprises: a timer for measuringelapsed time; a receiver for (i) receiving signals from at least onelight detector at a plurality of different times measured by the timerand (ii) receiving signals based on the detected ambient light level; acontroller for controlling at least one light emitter to emit lightduring at least one of the plurality of times during whichlight-detection signals are received; a transformer for producing thecorrected first-wavelength measurement and the correctedsecond-wavelength measurement based on signals received by the receiver;and a signal analyzer for determining the presence of smoke particlesoutside the housing.
 24. The smoke detector of claim 23, wherein thecontroller controls at least one light emitter to not emit light duringat least one other of the plurality of times during whichlight-detection signals are received.
 25. The smoke detector of claim 1,wherein the evaluation circuit is configured to: acquire a measurementof light including the first wavelength originating outside the housingwhile emitting light of approximately the first wavelength with at leastone said light emitter; acquire a measurement of light including thesecond wavelength originating outside the housing while emitting lightof approximately the second wavelength with at least one said lightemitter; and detect an ambient light level outside of the housing. 26.The smoke detector of claim 25, wherein the evaluation circuit isconfigured to: produce the corrected first-wavelength measurement atleast in part by adding to the measurement of light including the firstwavelength a first offset based on a function of the detected ambientlight level; and produce the corrected second-wavelength measurement atleast in part by adding to the measurement of light including the secondwavelength a second offset based on a function of the detected ambientlight level.
 27. The smoke detector of claim 25, wherein the evaluationcircuit is configured to acquire a first unilluminated measurement oflight including the first wavelength and the second wavelengthoriginating outside the housing without emitting light of approximatelythe first wavelength or light of approximately the second wavelengthfrom the one or more light emitters.
 28. The smoke detector of claim 27,wherein the evaluation circuit is configured to: produce the correctedfirst-wavelength measurement by (i) subtracting from the measurement oflight including the first wavelength the first unilluminated measurementof light including the first wavelength and the second wavelength and(ii) adding to the measurement of light including the first wavelength afirst offset based on a function of the detected ambient light level;and produce the corrected second-wavelength measurement by (i)subtracting from the measurement of light including the secondwavelength the first unilluminated measurement of light including thefirst wavelength and the second wavelength and (ii) adding to themeasurement of light including the second wavelength a second offsetbased on a function of the detected ambient light level.
 29. The smokedetector of claim 4, wherein the first threshold corresponds to a signallevel smaller than a signal level generated via smoke obscurationoutside the housing of approximately 4%/foot.
 30. The smoke detector ofclaim 1, wherein the evaluation circuit is configured to determine thepresence of an obstruction outside the housing based on at least one ofthe corrected first-wavelength measurement or the correctedsecond-wavelength measurement.
 31. The smoke detector of claim 1,wherein the one or more light emitters comprise a broadband light sourceemitting light over a range of wavelengths, the first and secondwavelengths being within the range of wavelengths.
 32. The smokedetector of claim 31, wherein the broadband light source comprises awhite light-emitting diode.